Touch this pad when popping popcorn in your microwave oven. The oven's sensor will tell the oven how long to cook depending on the amount of humidity it detects from the popcorn. See page 31 for more information.

Page 31 clarifies:

POPCORN lets you pop commercially packaged microwave popcorn.

(As opposed, I suppose, to popping kernels in a paper bag, which I also do.) Fine. Other microwave ovens have a similar button (though I must admit I haven't checked their manuals).

6 Answers
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A better instruction would be "do not rely entirely on the functionality of the popcorn button on your microwave, since microwaves vary widely as do bags of popcorn." But that's longer, and kind of complicated, so they abbreviate it "do not use popcorn button".

There's no problem with the power setting of the popcorn button, only with the timing. You should hang around near the microwave, and when you hear the pops slowing, stop it, no matter what your popcorn button thinks. And if you get a lot of unpopped kernels, and the popping didn't slow before the button said the microwave was done, consider doing it yourself next time and letting it pop for a little longer.

The problem with leaving it in too long is that the corn scorches (usually from the centre out) and the whole bag is inedible. If the energy is no longer going into popping kernels, it's going in to scorching what you already have. Get it out before that happens.

Most older microwaves do not have a humidity sensor; in these microwaves, the Popcorn button is a simple pre-programmed timer. Some allow you to adjust the time by 10-second increments as you use it and will remember the new value, but even cheaper ones simply come pre-programmed. The manufacturer is simply saying not to cook popcorn by time. I'd venture a guess that the manufacturer chose this wording because one too many customers complained that their product burned and stopped buying a particular brand because of it.

However, since your microwave uses a sensor to pop popcorn it should be ok to use. I suppose the popcorn bag could say "Do not use the popcorn button unless your microwave is fancy and has a humidity sensor" but since most microwaves do not have this sensor, most people probably wouldn't know even if it did, and popcorn pops just fine listening the "old-fashioned" way, they have gone with the simple wording. Sensor prices are coming down and more microwaves are becoming equipped with temperature and humidity sensors, so you this wording might change in the future. Or not, seeing how every coffee cup these days has the "Caution: Hot" warning on it...

I suggest trying your button a few times just to make sure it works as you expect, then you're probably safe to use it.

If the microwave-button works by measuring the humidity, it might rely on the permeability of the bag and doesn't work for bags that are more impermeable than other ones (some of them seem to have a coating inside).

I'd recomment to build on the popcorn bags hint, but maybe you should just give your popcorn-button a try. In that case, you should stay near the microwave to interrupt it, if it doen't stop when you think it should.

+1. Certainly possible. Do you know of commercial microwaveable popcorn with a more permeable bag than others'? Does it lack the "don't use the 'popcorn' button" instruction?
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msh210Apr 3 '12 at 20:39

Doubtful that the permeability of the bag has much to do with it -- commercial popcorn bags always let steam escape from the top as soon as a little pressure builds up.
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CalebApr 5 '12 at 3:08

My microwave has a popcorn button that then asks you to select the size (in ounces) of the popcorn you're about to pop. It's remarkably accurate for the 'standard' ~3.5oz bag, but would burn the little single serve sizes. For my microwave at least, it seems it's a simple time/power setting depending on the size of the bag.

I did a google search about how this popcorn function worked and it came up with an answer that said a microphone circuit is used. It went so far as to show the patent, circuits and an in-depth explaination as to how it worked. It seems that it would take the place of you listening for the popping to almost stop.

Nice find, I wouldn't have thought of that as being a method. If it's google.com/patents/US4952766 you're thinking of you could edit the link and patent number into your answer as a reference.
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PeterJNov 7 '13 at 9:21

3

The existence of a patent doesn't meant that the major manufacturers have incorporated the technology into the their product lines; that would also be important information.
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SAJ14SAJNov 7 '13 at 9:36

You might like using your bags to avoid the scorching issue that @kate-gregory brought up. The commercial bags can scorch because they have special material that "absorbs" the microwave energy to produce a heating element in the bag, therefore, it acts like a little heater.

I pop corn in a brown bag and it doesn't scorch. It usually doesn't pop every kernel especially if my corn is not fresh. This is because my regular brown bag doesn't have that material to give a little heating boast to pop all the kernels.